The invention relates to photoresists containing soluble polymer precursors (prepolymers) carrying unsaturated radicals bound in an ester-like manner to carboxyl groups, monomers and photoinitiators for forming relief structures composed of highly heat-resistant polymers.
Photoresists for forming relief structures composed of highly heat-resistant polymers are used on a large scale in the production of electrical and electronic components, in etching resists and electroplating resists or in printing forms.
Photoresists of this type are applied to a substrate in the form of a layer or film, and the layer or film is dried and subsequently exposed through a negative original. A crosslinking takes place at the exposed points, as a result of which the solubility at those points is drastically reduced. The regions which were not exposed are removed by dissolving (development) or peeling apart. The photocured relief structures left behind may then be converted by postcuring at relatively high temperatures into highly heat-resistant polymers which withstand temperatures of 250.degree.-400.degree. C. without adverse effects, for example on edge acuity and resolution.
According to German Patent No. 2,308,830, such photoresists contain soluble polymer precursors, so-called prepolymers. These prepolymers are, for example, polyaddition or polycondensation products of carbocyclic or heterocyclic compounds containing two functional groups capable of polyaddition or polycondensation reactions, and also, at least partially, in the associated ortho or peri position, radiation-reactive radicals bound in an ester-like manner to carboxyl groups (hereinafter termed ring compounds I) and diamines, diisocyanates, bis(acid chlorides) or dicarboxylic acids which in each case contain at least one cyclic structural element (hereinafter termed ring compounds II). However, if the prepolymers cited in German Patent No. 2,308,830 are used, 8-15 minutes of exposure are necessary to ensure an adequate crosslinking of the photoresists containing the prepolymers. These exposure times cannot be appreciably reduced even by adding the usual photosensitizers.
In the patent of addition to German Patent No. 2,308,830, German Patent No. 2,437,348, the prepolymers in the photoresist are modified in that, instead of the radiation-reactive radicals bound in an ester-like manner to carboxyl groups specified therein, oxyalkyl acrylate or oxyalkyl methacrylate groups bound in an ester-like manner to carboxyl groups are used. This is said to achieve a reduction in the exposure times. However, even the use of such photoresists for the mass production of electronic components is critical owing to the exposure times which are still too long.
A further increase in the photosensitivity of photoresists and a shortening, associated therewith, of the exposure times is described in European Patent Application No. 0,047,184, in which a polymerizable polyfunctional acrylate compound (monomer) and an aromatic bisimidazole photoinitiator, optionally substituted by halogen on the nucleus, are added to a soluble amide ester prepolymer. Although these photoresists exhibit a good photosensitivity, the addition of bisimidazole photoinitiators has disadvantages. Thus, for example, bisimidazole compounds in the resist solution may result in undesirable premature polyimide formation in the solution, as a result of which the shelf life of the photoresist solution is considerably reduced.